FMS-like tyrosine kinase 3 (FLT3) is mutated in about one third of acute myeloid leukemia cases. The most frequent FLT3 mutations in acute myeloid leukemia are internal tandem duplication (ITD) mutations in the juxtamembrane domain (23%) and point mutations in the tyrosine kinase domain (10%). The most frequent kinase domain mutation is the substitution of aspartic acid at position 838 (equivalent to the human aspartic acid residue at position 835) with a tyrosine (FLT3/D835Y), converting aspartic acid to tyrosine. Even though both of these mutations constitutively activate FLT3, patients with an ITD mutation have a much poorer prognosis. It has been previously demonstrated that the FLT3/D835Y knock-in mice survive significantly longer than FLT3/ITD knock-in mice. The majority of these mice develop myeloproliferative neoplasms with a less-aggressive phenotype.
Secondary mutations in the tyrosine kinase domain (KD) is one of the most common causes of acquired clinical resistance to small molecule tyrosine kinase inhibitors (TKIs) in human cancer. Recent pharmaceutical efforts have focused on the development of “type II” kinase inhibitors, which bind to a relatively non-conserved inactive kinase conformation and exploit an allosteric site adjacent to the ATP-binding pocket as a potential means to increase kinase selectivity. Mutations in FLT3 are the common genetic alteration in patients with acute myeloid leukemia (AML) (TCGA, N Engl J Med. 2013, 368: 2059-74) and are primarily comprised of constitutively activating internal tandem duplication (ITD) mutations (of 1-100 amino acids) in the juxtamembrane domain, and to a lesser extent, point mutations, typically within the kinase activation loop. Secondary KD mutations in FLT3-ITD that can cause resistance to the highly potent type II FLT3 inhibitors, such as, quizartinib, which achieved a composite complete remission (CRc) rate of about 50% in relapsed or chemotherapy-refractory FLT3-ITD+AML patients treated in large phase II monotherapy studies (Tallman et al., Blood, 2013;122:494). An in vitro saturation mutagenesis screen of FLT3-ITD identified five quizartinib-resistant KD mutations at three residues: the “gatekeeper” F691 residue, and two amino acid positions within the kinase activation loop (D835 and Y842), a surprisingly limited spectrum of mutations for a type II inhibitor. Mutations at two of these residues (F691L and D835V/Y/F) were subsequently identified in each of eight samples analyzed at the time of acquired clinical resistance to quizartinib (Smith et al., Nature, 2012;485:260-3). This finding validated FLT3 as a therapeutic target in AML. The type II multikinase inhibitor sorafenib, which also has some clinical activity in FLT3-ITD+AML, is ineffective against all identified quizartinib resistance-causing mutants, in addition to other mutant isoforms (Smith et al.). The type I inhibitor crenolanib has been identified a type I inhibitor of quizartinib-resistant D835 mutants (Zimmerman et al. Blood, 2013; 122:3607-15); however, no FLT3 inhibitor has demonstrated equipotent inhibition of the F691L mutant, including the ABL/FLT3 inhibitor ponatinib, which was designed to retain activity against the problematic gatekeeper T3151 mutant in BCR-ABL (Smith et al., Blood, 2013; 121:3165-71).
Accordingly there is a long felt need for new FLT3 inhibitors that overcome the drawbacks of the FLT3 inhibitors known in the art.